Sensor interface for hostile environments

Abstract

An interface circuit for a sensor including: a first injection-locked oscillator having: a first input coupled to a sensor, a free-running oscillation frequency of the first injection-locked oscillator being controlled by a signal from the sensor; and a second input coupled to receive a synchronization signal at a reference frequency, the first injection-locked oscillator being adapted to generate an output signal at said reference frequency, the output signal being phase shifted with respect to the synchronization signal as a function of the signal from the sensor.

The invention claimed is: 
     
       1. An interface circuit for a sensor comprising:
 a first injection-locked oscillator having:
 a first input coupled to a sensor, a free-running oscillation frequency of the first injection-locked oscillator being controlled by a signal from the sensor; and 
 a second input coupled to receive a synchronization signal at a reference frequency, the first injection-locked oscillator being adapted to generate an output signal at said reference frequency, the output signal being phase shifted with respect to the synchronization signal as a function of the signal from the sensor; and 
 
 an output circuit, clocked by said synchronization signal or by a clock signal derived therefrom, and adapted to generate a digital output signal based on a phase difference between the output signal and the synchronization signal, or a further reference signal generated based on said synchronization signal. 
 
     
     
       2. The interface circuit of  claim 1 , wherein the output circuit is clocked by said clock signal having a frequency equal to a multiple of the frequency of the synchronization signal. 
     
     
       3. The interface circuit of  claim 1 , wherein the output circuit is adapted to generate the digital output signal based on a phase difference between the output signal and a further reference signal generated by a second injection-locked oscillator having:
 a first input for controlling a free-running oscillation frequency of the second injection-locked oscillator; and 
 a second input coupled to receive the synchronization signal. 
 
     
     
       4. The interface circuit of  claim 3 , wherein the first input of the second injection-locked oscillator is coupled to the sensor. 
     
     
       5. The interface circuit of  claim 4 , wherein the sensor comprises first and second differential outputs, the free-running oscillation frequency of the first injection-locked oscillator being controlled based on a first differential output signal of the sensor, and the free-running oscillation frequency of the second injection-locked oscillator being controlled based on a second differential output signal of the sensor. 
     
     
       6. The interface circuit of  claim 5 , wherein the output circuit further comprises:
 a first counter adapted to determine a phase offset between the output signal of the first injection-locked oscillator and the synchronization signal; 
 a second counter adapted to determine a phase offset between the output signal of the second injection-locked oscillator and the synchronization signal; and 
 a calibration circuit configured to adjust the free-running oscillation frequencies of both the first and second injection-locked oscillators based on a comparison between an average of the first and second phase offsets and a reference phase offset. 
 
     
     
       7. The interface circuit of  claim 3 , wherein said output circuit comprises:
 a phase comparator; and 
 a control circuit having an output coupled to the first input of the second injection-locked oscillator and adapted to generate a control signal for controlling the free-running oscillation frequency of the second injection-locked oscillator based on the phase comparison. 
 
     
     
       8. The interface circuit of  claim 7 , further comprising a sigma-delta modulator coupled to the output of the phase comparator, and a further comparator coupled to the output of the sigma-delta modulator, wherein the control circuit is adapted to generate the control signal based an output signal of the further comparator. 
     
     
       9. The interface circuit of  claim 1 , wherein said output circuit comprises:
 a phase comparator; and 
 a control circuit having an output added to the first output signal of the sensor, the free-running oscillation frequency of the first injection-locked oscillator being controlled based on the sum of the first output signal and the control signal. 
 
     
     
       10. The interface circuit of  claim 1 , wherein the output circuit comprises a counter adapted to generate said digital output value by incrementing a count value between a first edge of the output signal of the first injection-locked oscillator and a first edge of the synchronization signal or of a further reference signal. 
     
     
       11. The interface circuit of  claim 1 , wherein the first injection-locked oscillator comprises:
 an oscillator comprising first and second capacitors charged or discharged based on a first current signal generated based on said first output signal of the sensor; and 
 an injection locking circuit adapted to charge or discharge the first and second capacitors based on the synchronization signal. 
 
     
     
       12. The interface circuit of  claim 11 , wherein the first injection-locked oscillator further comprises a flip-flop adapted to be clocked based on a voltage across the first capacitor and reset based on a voltage across the second capacitor. 
     
     
       13. The interface circuit of  claim 1 , comprising a single oscillator, the single oscillator generating the synchronization signal. 
     
     
       14. The interface circuit of  claim 13 , wherein the synchronization signal is the only timing signal used by the interface circuit. 
     
     
       15. The interface circuit of  claim 1 , wherein the sensor outputs to the first input a voltage or current level representative of a value sensed by the sensor, wherein an oscillation phase of the first injection-locked oscillator is controlled based upon the voltage or current level. 
     
     
       16. A method of outputting a sensor signal comprising:
 controlling a free-running oscillation frequency of a first injection-locked oscillator by a signal from a sensor coupled to a first input of the first injection-locked oscillator; 
 locking the frequency of an output signal of the first injection-locked oscillator to a reference frequency by applying a synchronization signal at the reference frequency to a second input of the first injection-locked oscillator, the output signal being phase shifted with respect to the synchronization signal as a function of the signal from the sensor; and 
 generating, by an output circuit clocked by said synchronization signal or by a clock signal derived therefrom, a digital output signal based on a phase difference between the output signal and the synchronization signal, or a further reference signal generated based on said synchronization signal.